Electric heater



R. COX ET AL ELECTRIC HEATER May 13, 1952 Filed July 11, 1950 FIG.3

FIG. 4-

FIGJ

was

INVENTORSZ FIG.Z

R COX THEui ATTORNEY Patented May 13, 1952 ELECTRIC HEATER Robert Coxand Hendrik Sipkes, Ijmuiden, Netherlands, assignors to ShellDevelopment Company, San Francisco, Calif., a corporation of DelawareApplication July 11, 1950, Serial No. 173,078 In the Netherlands July19, 1949 6 Claims. 1

This invention relates to electric heaters and pertains moreparticularly to a device for electrically heating a gas flowingtherethrough.

Electric heaters presently used for heating a flowing gas generallyemploy a heating element or conductor having a constant cross-sectionalarea throughout its length. With the conductor arranged lengthwise inthe heater in a plane parallel to a gas stream passing therethrough, itis readily apparent that diiiiculties in heating the gas will beencountered because of the inefficiency of the above arrangement. Thus,with a constant flow of cold gas being put through the heater when aconstant voltage is applied to the conductors or heating elements, theconductors will have a much lower temperature at one end where the coldgas is entering the heater than at the other end where the conductor isin contact with the warm gas leaving the heater.

Since the material of which a conductor is made has a maximumtemperature to which it can be heated without destroying or deformingthe conductor, it may be seen that the temperature at the cold end ofthe conductor cannot rise above a certain value which is considerablylower than the temperature of the warm end. Hence, the use of conductorsof constant cross-sectional area in a heater limits the amount of heatwhich may be developed, calculated on a given weight of the conductor.Since the temperature at the cold end is much lower than the maximumtemperature which'the material of the conductor can stand, neither thematerial nor the space occupied by the conductor is used economically.

It is therefore a primary object of the present invention to provide anelectric heater employing heating elements or conductors of specialshape whereby the temperature difference between any two points alongthe conductors is made relatively small when the stream of gas is passedthrough the heater in a direction parallel to the conductors. Otherobjects of this invention will be understood from the followingdescription taken with reference to the attached drawing, wherein:

Figure 1 is a longitudinal view, of a gas. heater according to the tion.7

in cross section, present inventrical gas heater employing longitudinalconductor strips, the resistance per unit of length of which decreasesin the direction of the flow of gas. Thus, the heat developed per unitlength is greater where the gas is cold than where the conductor is-incontact with the already heated gas. Provided that the conductor is ofhomogeneous material, the resistance per unit of length decreases in thedirection of the flow of gas if the cross-sectional area increases inthe same direction. The cross-sectional area can be altered by varyingthe width and/or the thickness of the strip; in practice, only variationof the width is practicable. If the width increases, the heating surfaceis increased proportionally, whereas variation of the thickness hashardly any effect on the size of the heating surface. To

should be as great as possible, and in connection therewith thethickness should be kept as small as is consistent with the requiredstrength and rigidity of the conductor; the most favorable thicknessshould preferably not be departed from. With appropriate gradation ofthe resistance per unit of length, the cross-sectional area and thewidth of the strip respectively, an almost uniform temperature can beachieved over the whole length of the conductor when put to the use forwhich the device is intended.

In order to maintain accurately a constant temperature over the wholelength of the stripshaped conductor, it is necessary to use shapes withslightly curved outlines, for example, such as given by hyperbolas.These lines can be determined by calculation. As they differ onlyslightly from straight lines, strips in the form of elongated trapezoidscan be used in most'cases.

An electrical gas heater maximum permissible temperature for thematerial of which the conductor is made is appreciably lower at thepoint of heaviest load (i. e., at the upper end of a suspendedstrip-conductor) than elsewhere in the conductor. By employing a heaterof the present design where the cold gas contacts the warmest part ofthe conductor and the warm gas contacts the coldest part of theconductor, an advantage is realized by the increased transfer of heat tothe cold gas which permits the use of a heater having conductors whichare shorter in length and hence lighter in weight.

As shown in' Figures 1 and 2, a preferred embodiment of the presentheater may comprise a' longitudinal housing or tube H closed at eitherend by plates or covers 12 and I3, which may be secured to the housing Il in any suitable manner as by bolts l4. Both covers H and [2 areprovided with suitable port means, .suchas flanged nipples I5 and IE, towhich may be connected the intake and discharge flow lines (not shown)serving as a gas conduit.

One cover l2 (preferably the top cover when the axis of housing II ispositioned vertically) is provided with a central opening I"! in which abar member I8 is fixedly held in a fluidtight manner, said bar memberserving. as an electrical conductor. One end of'the'bar member, [8extends into the housing ll through apair'of flanged insulation rings:l9" and: of porcelain, rubber, etc., which-serve as electricalinsulationbetween the bar I8 and the'cover'l2'. Aholding ring '2! and aholding'nut'zrmay'be secured to the bar member 18, as by weldingor'screw threads, and serveto position the bar [8 with'regardto thecover l2.

Aflixed'totheendoithe'bar member I8 within the'housingd l 'isa supportring or spider 23 from which depend 'a'plurality of'metallic finelements or"strip; conductors; The conductors 24 are preferably,secured'to the spider'23 by welding. The"spider*23, together with asecond spider 25 welded to"the-lower' ends ofthe conductors 24, servesas 'means for maintaining 1 the conductors in preferably equidistantspaced relationship to each other. If desired, additional spiders orspacing elements maybe secured to the conductors at-other' pointsbetween the ends thereof.

The conductors 24 are preferably made of any suitable'thingauge metal,the longsides of said conductors being tapered as shown in Figure 1.Thenarrow-ingof strips 25 can also be carried out in-steps as shown inFigure 3 instead of being gradually-tapered. Increased resistance perunit length can also be "obtained by'notch'es orper forations 21 0fincreased depth ina strip 28, as shown" in Figure '4: The stripconductors 2 4 and 28 mayals'o be-provided with slots 29 andeil' whichare-adapted to receive'the'support spiders 23 and-'25; respectively.

Themost satisfactory method of suspending thestrip-conductors is withthe broadend uppermost; ,the'greatest mechanical loadis then borneby'the strongest' parts. Inthis case the gas to beheated must bepassedaiongthe conductors from bottom to top. Should'the'strength ofconstruction permit, the-strips may, however, also be suspended fromtheir thin ends; and the gas passed from top to bottom.

The-strips 24" which are to be fitted into a tube can be-distributed invariousways over the available space. A preferable, form ofconstructionisthat'whereby-a number'of strips are bent toa 'v sh'ape i.e. bent insuch' awaythat a cross;- section at right angles to the" axisof the tube shows a V-shapa-as-shown" in Figure 2; The

apexes of the Vs should preferably point towards the axis of the tube.To ensure as far as possible an even distribution of heat, the bentstrips 24 should be spaced in such a manner that roughly the same amountof gas flows along the outer sides of the conductors as along the innersides.

The spider 25, which is fixedly secured to the lower end of theconductor-strips 24, is also preferably secured to a bar member 3! whichextends through a central opening 32 in the bottom cover plate 13 of thehousing H. A suitable stuffing box 33 surrounds the bar member 3| toprovide a fluidtight seal which allows the bar 31 to slide in or outdueto expansion or contraction of the strip-conductors 24 as they areheated and cooled. Since the member 3| serves as an electrical contactbar or terminal for supplying current to the conductors 24, it iselectrically insulated from the coverplate I3 in any suitable manner, asby a surrounding tube of insulation 34 which may be carried in arecessed portion of the bar member, as shown in Figure 1. If desired,the entire housing H may be insulated to prevent excessive heat losses,as by installing suitable insulating material 35 around the inside oroutside of the housing ll.

While in the embodiment illustrated in Figure l the-lower bar member 3|is slidably mounted in the-lower plate [3, it is realized that variousdesign changes may be made in the present heater to allowfor-theexpansion of the strip-conductors 24, without departing from the scopeof the invention. Thus, instead of the bar member 3ibeingfixedly'secured to the spider 25 and mounted'ior. sliding movementthrough the lower cover plate 13, a bar member 36'(Fig. 5) may befixedly secured to the lower plate l3 by holding nuts 3'! and'38 -and byinsulation rings 39 and' lil while a spider 4| is mounted for slidingelectrical contact along the portion of the bar 35 which extends intothe housing I l. Hence, as the strip-conductors -24-expand and contractthe spider 4i slides along the bar member 36 which is mounted in afluidtight manner in plate I3 In operation, the intake and outletnipples l5 and I6 are connected into a pipe line carrying gastobe-heated and the contact bars l8 and 31 areconnected to a suitablepower source (not shown) whereby electric current may be applied tothestrip-conductorsM. Gas entering the inlet port- 15' flows past thehottest and smallest portions of the strip-conductors 24 first andcontinues upward in the housing H past the wider and cooler portions ofthe conductors and thence out the discharge port [5. The conductors 24in=the form of strips are particularly suitable for heating purposesowing to their broad surface and relatively small cross-sectional area.When strip conductors 24 of'a suitable taper are employed, asubstantially uniform temperature may be achieved over the whole lengthof the conductors.

The present gas heater has been found to be especially effective whenused to pre-heat a mixture of nitrogen and hydrogenbefore the mixture-isbrought into contact with acatalyst duringthe synthesis of ammonia.While the heat of the reaction is generally used to pre-heat the gases;it is necessary to use an auxiliary heater during the start-up period.

By way of example, a few figures are given below relating to thedimensions of a device according-to" the invention used, inanammoniasynthesis'column'wherein a quantity of 'nitrogenhydrogen mixturein the proportion of 3:1 is brought by the heating device to atemperature suitable for the reduction of a fresh oven charge:

Length of heating device 3.20 m. Quantity of gas to be heated 1100mfi/hour Pressure of the gas 50 atm. Diameter of heater 90 mm. Number ofstrip-conductors (material: V2A steel; trapezoidshaped; arranged as inFigure 2; space between strips and wall of tube equal in size to spacein the centre between the strips; the strips bent at an angle of 60) 6Width of strips, upper end about 67 mm.

By the arrangement described above, the temperature differential betweenthe ends of the conductor strips is reduced by approximately a factor often as compared with uniform width strips.

The invention claimed is:

1. An electrically operated gas heater comprising a closed housinghaving a chamber formed therein, inlet and outlet port means at oppositeends of said housing for flowing a gas through said chamber, a pluralityof metal conductors carried within said chamber, said conductorsextending over a major portion of the distance between the inlet andoutlet port means, the resistance per unit length of said conductorsdecreasing and the cross-sectional area thereof increasing in thedirection of the flow of gas through said chamber, and means extendingthrough the wall of said housing for supplying an electric current tosaid conductors.

2. A device for electrically heating a flowing gas, said devicecomprising a closed housing having a chamber formed therein, inlet portmeans provided near the bottom of said housing, outlet port means nearthe top of said housing, a plurality of metal conductors suspendedvertically within said housing, the cross-sectional area of saidconductors taken in a horizontal plane in-- creasing from the bottom tothe top of said conductors, and means extending through the wall of saidhousing for supplying an electric current to said conductors.

3. A device for electrically heating a flowing gas, said devicecomprising a closed housing having a chamber formed therein, inlet portmeans provided near the bottom of said housing, outlet port means nearthe top of said housing, a plurality of metal conductors suspendedvertically within said housing, said conductors comprising strips ofsheet metal in the form of elongated trapezoids, the width of saidconductors decreasing from the top to the bottom thereof, and meansextending through the wall of said housing for supplying an electriccurrent to said conductors.

4. A device for electrically heating a flowing gas, said devicecomprising a closed housing having a chamber formed therein, inlet portmeans provided near the bottom of said housing, outlet port means nearthe top of said housing, a plurality of metal conductors suspendedvertically within said housing, said conductors comprising strips ofsheet metal in the form of elongated trapezoids, the width of saidconductors decreasing from the top to the bottom thereof, supportmembers within said housing securing said conductors to the top andbottom of said housing, and electrical contact bar means extendingthrough the wall of said housing and in contact with said support meansfor supplying an electric current to said conductors.

5. .A device for electrically heating a flowing gas, said devicecomprising a closed housing having a chamber formed therein, inlet portmeans provided near the bottom of said housing, outlet port means nearthe top of said housing, a plurality of metal conductors suspendedvertically within said housing, said conductors comprising strips ofsheet metal in the form of elongated trapezoids, the width of saidconductors decreasing from the top to the bottom thereof, and electricalcontact bar means extending through the wall of said housing and incontact with said support means for supplying an electric current tosaid conductors, one of said support members being mounted on one ofsaid bar means in movable relationship with one end of said housing.

6. A device for electrically heating a flowing gas, said devicecomprising a closed housing having a chamber formed therein, inlet portmeans provided near the bottom of said housing, outlet port means nearthe top of said housing, a plurality of metal conductors suspendedvertically within said housing, said conductors comprising strips ofsheet metal in the form of elongated trapezoids, the width of saidconductors decreasing from the top to the bottom thereof, saidconductors being bent along their vertical center lines to form V-shapedtroughs having apexes pointing toward the center of said housing, andmeans extending through the wall of said housing for supplying anelectric current to said conductors.

ROBERT cox. HENDRIK SIPKES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,318,028 Thomson Oct. 7, 19191,332,852 Levy Mar. 2, 1920 1,738,164 Zingg Dec. 3, 1929 2,070,129Ireland Feb. 9, 1937

